1. Field of the Invention
The present invention relates to an electrolyte for a lithium ion secondary battery and a lithium ion secondary battery including the same. More particularly, the present invention relates to an electrolyte for a lithium ion secondary battery, which provides improved safety under overcharge and high-temperature storage conditions, and a lithium ion secondary battery including the same.
2. Description of the Related Art
Recently, as the electronic industry has advanced, technical research into portable and wireless electronic instruments including telephones, video cameras and personal computers has progressed rapidly. Accordingly, as a drive source for these instruments, a secondary battery having a small size, low weight and high energy density is increasingly in demand. Particularly, a non-aqueous electrolyte-based secondary battery is greatly expected to serve as a battery having high voltage and high energy density, wherein the non-aqueous electrolyte-based secondary battery uses a lithium-containing metal oxide providing a voltage of about 4V as a cathode active material and a carbonaceous material capable of lithium intercalation/deintercalation as an anode active material.
Such lithium ion secondary batteries generally use carbonaceous materials as an anode active material and metal oxides such as LiCoO2 as a cathode active material. Additionally, a porous polyolefin-based separator is inserted between an anode and a cathode, and then a non-aqueous organic solvent containing a lithium salt added thereto is injected to complete the manufacture of a battery. During charge cycles, lithium ions are deintercalated from cathode active materials and then intercalated into carbonaceous layers of anodes. On the contrary, during discharge cycles, lithium ions are deintercalated from anode active materials and then intercalated into cathode active materials.
Lithium ion secondary batteries have an average charge/discharge voltage of about 2.7 to 4.2V and thus can provide relatively high electric power compared to other alkali batteries, Ni-MH batteries, Ni—Cd batteries, etc. However, in order to obtain such a high drive voltage level, required is an electrolyte composition electrochemically stable in the charge/discharge voltage range of lithium ion secondary batteries. Such electrochemically stable electrolytes include non-aqueous organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, etc.
Lithium ion secondary batteries using such non-aqueous organic solvents have high voltage and high energy density as well as show excellent shelf stability and low-temperature quality, and thus are widely used in portable electric products.
However, most non-aqueous organic solvents have a low flash point and high flammability. Therefore, when a battery is overcharged, lithium may be precipitated excessively at a cathode and intercalated excessively at an anode depending on charging conditions, thereby causing the cathode and anode to be in a thermally unstable state. As a result, rapid exothermic reactions may occur due to the decomposition of organic solvents used in electrolytes. Additionally, the so-called thermal runaway phenomenon may occur and the battery may be exploded or ignited. In other words, there is a serious problem related with poor battery safety.